Shaft damping arrangement

ABSTRACT

Imbalance vibration of each of two concentric rotating shafts is damped simultaneously by a single damped bearing. By supporting one of the shafts on a damped bearing which is fixed to ground, and by transmitting imbalance vibration from the other shaft into the first shaft by means of a nondamped intershaft bearing, the imbalance vibration of both shafts is damped by the single damped bearing.

United States Patent 1 [111 3,756,672 Hibner et al. 1 Sept. 4, 1973 [54]SHAFT DAMPING ARRANGEMENT 3,473,853 10/1969 Goss 308/26 3,491,536 1/1970Hadaway 308/189 R [75] lnvemm- Dav"! Colcheste'v C011", 3,531,167 9/1970Edge et a1 308/187 Roger J. Comeau, Ware, Mass. [7 3] Assignee: UnitedAircraft Corporation, East Primary Examiner-Charles J. Myhre Hartford,Conn. Assistant Examiner-R. H. Lazarus Filed: y 1972 Attorney-Charles A.Warren [21] Appl. No.: 256,797 57 ABSTRACT Imbalance vibration of eachof two concentric rotating [52] US. Cl. 308/26, 308/189 Shafts is dampedsimultaneously by a single damped [51] Int. Cl. Fl6c 27/00 bearing. Bysupporting one of the shafts on a damped [58] Field of Search 308/15,26, 189, bearing which is fixed to ground, and by transmitting 308/187imbalance vibration from the other shaft into the first shaft by meansof a nondamped intershaft bearing, the [56] References Cited imbalancevibration of both shafts is damped by the sin- UNITED STATES PATENTS gledamped bearing 3,357,757 12/1967 Morley 308/26 11 Claims, 8 DrawingFigures ZZ 4Z Z SHAFT DAMPING ARRANGEMENT BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to damping shaftvibration, and more particularly to damping shaft vibration of twoconcentric rotating shafts using a single damping means.

2. Description of the Prior Art Where two concentric shafts havecritical speeds within their operating ranges, these shafts must usuallybe clamped to prevent failure of the supporting structure due to highvibratory stress and possible shaft failure due to large shaftdeflections. Generally, the shafts are supported in a manner requiringthe use of separate damping means for each shaft. The damping means isoften one or more judiciously placed damped bearings somewhere along thelength of each shaft. In gas turbine engines oil damped bearings areoften utilized because of their effectiveness, but they are expensive,as are all damped bearings as compared to nondamped bearings.

SUMMARY OF THE INVENTION One object of the present invention is toreduce the number of damped bearing means necessary to damp whirl motionof each of two concentric shafts.

More particularly, it is an object of the present invention to provide abearing and support arrangement for two concentric shafts adapted toallow the use of a single damped heating means to simultaneously dampwhirl motion of each of the shafts.

Accordingly, concentric shafts form an annular passageway therebetween,one of said shafts having a span radially supported from a span of theother shaft through an intershaft nondamped bearing, the supporting spanof the other shaft being disposed on nonrotating support means throughdamped bearing means. Vibration of the supported span is transmittedinto the supporting span through the intershaft bearing whereupon it isdamped by the damped bearing means along with any independent vibrationof the supporting span.

This bearing and support arrangement for concentric shafts is sometimesknown as a piggy-back design since one of theconcentric shafts isradially supported or hung from'the other shaft. Hanging one span fromthe other permits a large transmission of energy from the supported spaninto the supporting span; additionally, disposing the supporting span onthe damped bearing means increases the amount of transmitted energy thatthe damped bearing means is likely to see, resulting in more effectivedamping of the supported span.

This invention has particular application in gas turbine engines, and isparticularly adapted to use with an oil damped bearing having thecharacteristic wherein the entire radial load from the supporting shaftis transmitted into the nonrotating support means through an oil film.This type of oil damped bearing is able to damp many modes of vibrationfrom both shafts individually and simultaneously since the spring rateand the damp-,

ing constant of the oil film is determined by the amount of oilcompression alone in contrast, for example, to being determined by amechanical spring and oil film in parallel.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of the preferred embodiment thereof as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial elevation view,partly in section of a gas turbine engine embodying the presentinvention.

FIG. 2 is a schematic view, graphically illustrating critical speeddisplacement of the high rotor shaft of the engine shown in FIG. 1.

FIG. 3 is a schematic view, graphically illustrating critical speeddisplacement of the low rotor shaft of the engine shown in FIG. 1.

FIG. 4 is a sectional view more clearly showing the bearing arrangementin the turbine area of the engine shown in FIG. 1.

FIG. 5 is a schematic representation of a section taken along the line5-5 in FIG. 4, illustrating the ope'ration of an oil damped bearing.

FIG. 6 is a schematic representation of nonsynchronous whirl of a shaft.

FIGS. 7 and 8 are sectional views showing modifications of the bearingarrangement of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Consider, as an example of oneapplication for the present invention, the gas turbine engine 10 asshown in FIG. 1. The engine comprises a low rotor shaft 12 and a highrotor shaft 14. The low rotor shaft 12 is a long thin shaft extendingfor substantially the entire length of the engine 10; it is supportedalong its length at three locations: A thrust bearing 16 and a rollerbearing 18 near its forward end 20, and a roller bearing 22 near itsrearward end 24; the roller bearing 22 is a damped bearing. Thesebearings are in turn tied to ground through nonrotating supportstructure 26, 28 and 30, respectively. The high rotor shaft 14 isconcentric to the low rotor shaft 12 and outwardly spaced therefrom,forming an annular passageway 32 therebetween. The high rotor shaft isalso supported in three locations: A thrust bearing 34 tied tononrotating support structure 36 supports its forward end 38; a rollerbearing 40, tied to nonrotating support structure 42, provides supportat a more or less central position along the length of the shaft; andthe rearward end 44 of the high rotor shaft 14 is supported or hung fromthe low rotor shaft 12 by means of an intershaft bearing 46 positionedwithin the annular passageway 32. The span of high rotor shaft betweenthe roller bearing 40 and the rearward end 44 of the high rotor shaftcarries the high pressure turbine, generally represented by the numeral48. The span of the low rotor shaft between the roller bearings 18, 22carries the low pressure turbine, generally represented by the numeral50. As hereinbefore and hereinafter used, the term span" is defined tomean the portion of shaft extending between any two adjacent bearingstied to ground through nonrotating support structure, or the length ofshaft extending from a bearing tied to ground through nonrotatingsupport structure, to the end of the shaft, when no other such bearingintervenes therebetween. I

The engine 10 shown in FIG. 1 is designed such that the high and lowrotor shafts 12, 14 each have only one critical speed within theiroperating ranges; and at these critical speeds the maximum whirlamplitudes of the shafts l2, l4 occur in the spans carrying the highpressure turbine 48 and the low pressure turbine 50,

respectively. These spans are hereinafter referred to as the highturbine span and low turbine span, respectively. As will hereinafter bemade clear, the particular bearing and support arrangement for the highand low rotor shafts has been chosen to allow damping of these shafts attheir respective critical speeds by means of a single damper. In thisparticular application the critical speeds of each of these spans occursubstantially simultaneously, requiring the damper to be able toeffectively damp two different modes of vibration at the same time,although this may not be a requirement for other applications of thisinvention.

In accordance with the present invention, the central bearing 40 for thehigh rotor shaft is a nondamped bearing and the nonrotating supportstructure to which it is tied is relatively inflexible; on the otherhand, the rearward end 44 of the high rotor shaft is actually a free endin that it is not tied to an inflexible support structure. It isrelatively free to move in a radial direction. Thus, when the highturbine span whirls or vibrates as a result of the high rotor shaft 14reaching a critical speed, it will tend to pivot about the centralbearing 40. This is represented schematically in FIG. 2, where similarnumerals identify the corresponding features of FIG. 1. FIG. 2 shows thedisplacement of the high rotor shaft at the high rotor critical speed of11,000 revolutions per minute. Since the high turbine span is supportedfrom the low rotor shaft 12 through the intershaft bearing 46, the highturbine span whirl is actually transmitted through the intershaftbearing 46 into the low rotor shaft, imparting a whirl motion to the lowrotor shaft. By positioning the intershaft bearing 46 near the end 44 ofthe high turbine span, the greatest amount of whirling motion or energyis transmitted into the low turbine span. The damped bearing 22, at therearward end of the low rotor shaft, effectively damps the whirl motionimparted to the low rotor shaft 12 by the high rotor shaft 14, which, ofcourse, has the effect of damping the high rotor shaft itself.

FIG. 3 is a schematic representation of the low rotor critical speed of5,500 revolutions per minute. This critical speed is a result ofimbalance of the low turbine and, as stated results in a mode ofvibration wherein the low turbine span has the maximum whirl amplitudeof the low turbine shaft. This low turbine span mode of vibration isalso effectively damped by the damped bearing 22. As hereinbeforementioned, the low rotor and high rotor critical speeds occursubstantially simultaneously in the engine 10. Thus, the vibratorymotion of the low turbine span requiring damping by the damped bearing22 is the sum of the vibratory motion imparted by the high rotor shaftat its critical speed and the vibratory motion of the low rotor shaft atits critical speed.

In this exemplary embodiment an oil damped bearing is employed at 22 toaccomplish the damping, although other types of bearingdampers may alsobe effective such as a friction damper or an elastomeric damper; it isadvisable to repeat at this time that whatever damping means is chosen,it must be able to damp at least two different modes of shaftvibration-possibly, as in the present example, simultaneously.

The oil damped bearing 22 is best described with reference to FIG. 4.The bearing is more or less typical or roller bearings well known tothose skilled in the art and could just as well be a ball bearing. Itcomprises an inner race 52 fixedly attached to a low turbine rotorsupport 54 which is in turn splined at 56 to the low rotor shaft 12. Aplurality of rollers 58 are circumferentially spaced about the periphery60 of the inner race. An outer race 62 surrounds the rollers 60 and isin contact therewith. The nonrotating support structure 30 provides ahousing 64 for the outer race. The outer race 62 is radially spaced fromthe housing 64 forming an annular space 66 therebetween. Oil, from asource not shown, enters one end of the annular space 66 through aplurality of holes 69 in the housing; the oil exits through a pluralityof holes 70 in the housing at the other end of the annular space. Seals72 are provided at both ends of the outer race to prevent the loss ofoil and oil pressure within the annular space. Thus an oil film extendsover the entire axial length of the outer periphery 68 of the outerrace, the end 24 of the low turbine span being fully supported by thisfilm; in other words, all radial loads from the low rotor shaft 12 whichenter the damped bearing 22 are transmitted to the housing 64 throughthis oil film, which is on the order of 0.010 inch thick.

To better understand the operation of the damper, reference is now madeto FIG. 5, which is a schematic cross section through the housing 64,bearing 22, and low rotor shaft 12, wherein similar numerals correspondto similarly numbered features in FIG. 4. Assume for the purpose of theinstant discussion that only a single mode of shaft vibration is beingdamped. As is well known to those skilled in the art, shaft whirl motionis simply the center of the shaft 74 rotating in a circle about the axis76 of the engine, which in FIG. 5 is the center of the housing innerdiameter 78. The dotted line 80 represents the path which the center ofthe shaft 74 follows when whirling. This type of circular motion isknown as synchronous whirl. As the shaft whirls, the outer diameter 68of the outer race 62 shears and pumps the oil around the annulus 66(greatly exaggerated). This takes work or energy. The energy is takenfrom the whirling of the shaft, thus maintaining the shaft whirlamplitude at an acceptable level.

When the vibratory amplitude of a second shaft, such as the high rotorshaft 14 of the present engine configu ration, is added to the vibratoryamplitude of the shaft being directly damped, such as the low rotorshaft 12 of the present engine configuration, the motion of the center74 of the directly damped shaft is no longer a circle. With reference tothe schematic drawing shown in FIG. 6, the numeral 84 indicates theengine axis, and the dotted line 86 represents the type of path followedby the center of the low rotor shaft when the high rotor shaft whirl issuperimposed upon the low rotor shaft whirl. This motion is known asnonsynchronous whirl. Although those skilled in the art do not believethat an oil damped bearing of the type herein described can effectivelydamp nonsynchronous whirl, a rig constructed in accordance with thepresent invention has demonstrated otherwise.

As is apparent from the foregoing discussion, an oil damper of the typeshown schematically in FIG. 5 can only effectively damp shaft whirl whenthat whirl occurs at the point along the shaft axial length whichimparts radial motion to the bearing. In other words an oil dampedbearing depends on radial motion of the bearing with respect to thehousing to transfer vibratory energy from the shaft into the oil film.If a node occurs along the shaft length at the point where the bearingsupports the shaft or very close thereto, then there will be no radialmotion of the bearing and thus no transfer of energy from the shaft intothe oil film; in that instance the whirling motion of the shafts willnot be effectively damped. Also, certain critical speeds may cause anunacceptable whirl amplitude in one span of shaft that results in only asmall whirl amplitude in the span of shaft being supported by the dampedbearing. In that instance a second damped bearing may be requiredelsewhere along the shafts. It is important to understand that thisinvention does not preclude the use of such a second damped bearing; theinvention is in being able to damp at least one critical speed of eachshaft through a single damped hearing. The shafts may have othercritical speeds wherein almost all the shaft whirl energy is in a spanso far removed from the damped bearing that the bearing sees littlemotion and is ineffective.

From the foregoing principles and description of an exemplary embodimentof the present invention, it should be apparent that various otherbearing and support arrangements for concentric shafts, embodying allthe features of the present invention, may be devised such that only asingle damper may be utilized to damp vibrations from both shafts.

As one example of an alternative method for supporting and damping theshafts of an engine configuration similar to that shown in FIG. 1,reference is made to FIG. 7. By comparing FIG. 7 with FIG. 5 it is notedthat the only change is placing the damped bearing and its nonrotatingsupport structure inside the low rotor shaft. Primed numerals in FIG. 7correspond to similarly numbered features of FIG. 5.

FIG. 8 shows a somewhat different arrangement wherein the damped bearingand nonrotating support structure directly support the rearward end ofthe high turbine span. The low turbine, span of the low rotor shaft issupported by the high rotor shaft through the intershaft bearing. InFIG. 8 double primed numerals correspond to similarly numbered featuresof FIG. 5. In this arrangement low turbine span whirl is transmittedthrough the intershaft bearing 46" into the high rotor shaft 14"whereupon it is damped by the damped bearing 22" along with whirl of thehigh turbine span. It is not advisable in this instance to axially alignthe intershaft bearing with the damped bearing, for in that situationthere would be more of a likelihood that a node will occur at the axiallocation of the damped bearing.

Although the invention has been shown and described with respect topreferred embodiments thereof, it should be understood by thoseskilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and scopeof the invention.

Having thus described typical embodiments of our invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:

1. A bearing and support arrangement for concentric shafts comprising:

nonrotating support means;

damped bearing means disposed on said support means;

first shaft means having at least one span, one end of said span beingrotatably disposed on said damped bearing means and being radiallysupported thereby;

second shaft means concentric whith said first shaft means and formingan annular passageway therebetween; and

nondamped bearing means disposed on said first shaft span and withinsaid annular passageway, said second shaft means having a span disposedon said nondamped bearing means, and hung from said first shaft spanthrough said nondamped bearing means, said nondamped bearing beingadapted to transmit whirling motion of said second shaft span into saidfirst shaft span, said damped bearing means being adapted to dampwhirling motion of said first shaft span and of said second shaft span,said damped bearing being the only damped bearing means for dampingwhirling motion of said spans.

2. The bearing and support arrangement according to claim 1 wherein saiddamped bearing means is an oil damped bearing means including an annulusof oil, and said end of said first shaft span is fully supported by saidannulus of oil.

3. The bearing and support arrangement according to claim 1 wherein saiddamped bearing means is axially spaced from said nondamped bearingmeans.

4. The bearing and support arrangement according to claim 1 wherein saidfirst and second shaft means are gas turbine engine shafts.

5. The bearing and support arrangement according to claim 1 wherein saidsecond shaft span has a free end, and is hung at its free end from saidfirst shaft span through said nondamped bearing means.

6. In an engine including a low rotor shaft and a high rotor shaft, thehigh rotor shaft being concentric with the low rotor shaft and outwardlyspaced therefrom forming an annular passageway therebetween, the highrotor shaft having a span with a free end and the low rotor shaft havingat least one span, a bearing and support arrangement for the low andhigh rotor shafts comprising:

nonrotating support means;

damped bearing means disposed on said support means, the end of the lowrotor span being rotatably disposed on said damped bearing means;nondamped bearing means positioned within the annular passageway, saidlow rotor span being rotatably disposed on said nondamped bearing means,and the free end of the high rotor span being rotatably disposed on saidnondamped-bearing means and hung from said low rotor span therethrough,

' said nondamped bearing means adapted to transmit whirling motion ofsaid high rotor span into said low rotor span, said damped bearing meansadapted to damp whirling motion of said high and low rotor spans, andsaid damped bearing means being the only damped bearing means fordamping the whirling motion of both spans.

7. The bearing and support arrangement according to claim 6 wherein saiddamped bearing means is an oil damped bearing means including an annulusof oil, and

said end of said low rotor span is fully supported by said annulus ofoil.

8. The bearing and support arrangement according to claim 6 wherein saidlow rotor shaft has a critical speed within its operating range and saidhigh rotor shaft has a critical speed within its operating range, andsaid damped bearing means is the only damped bearing means for dampingthe whirling motion of said high and low rotor spans at said criticalspeeds. 9. In an engine including a low rotor shaft and a high rotorshaft, the high rotor shaft being concentric with the low rotor shaftand outwardly spaced therefrom forming an annular passagewaytherebetween, the low rotor shaft having a span with a free end and thehigh rotor shaft having at least one span, a bearing and supportarrangement for the low and high rotor shafts comprising:

nonrotating support means; damped bearing means disposed on said supportmeans, the end of the high rotor span being rotatably disposed on saiddamped bearing means; nondamped bearing means positioned within theannular passageway, said high rotor span being rotatably disposed onsaid nondamped bearing means, and the free end of the low rotor spanbeing rotatably disposed on said nondamped bearing means and hung fromsaid high rotor span therethrough,

said nondamped bearing means adapted to transmit whirling motion of saidlow rotor span into said high rotor span, said damped bearing meansadapted to damp whirling motion of said high and low rotor spans, andsaid damped bearing means being the only damped bearing means fordamping the whirling motion of both spans.

10. The bearing and support arrangement according to claim 9 whereinsaid damped bearing means is an oil damped bearing means including anannulus of oil, and said end of said high rotor span is fully supportedby said annulus of oil.

11. The bearing and support arrangement according to claim 9 whereinsaid low rotor shaft has a critical speed within its operating range andsaid high rotor shaft has a critical speed within its operating range,and said damped bearing means is the only damped bearing means fordamping the whirling motion of said high and low rotor spans at saidcritical speeds.

1. A bearing and support arrangement for concentric shafts comprising:nonrotating support means; damped bearing means disposed on said supportmeans; first shaft means having at least one span, one end of said spanbeing rotatably disposed on said damped bearing means and being radiallysupported thereby; second shaft means concentric whith said first shaftmeans and forming an annular passageway therebetween; and nondampedbearing means disposed on said first shaft span and within said annularpassageway, said second shaft means having a span disposed on saidnondamped bearing means, and hung from said first shaft span throughsaid nondamped bearing means, said nondamped bearing being adapted totransmit whirling motion of said second shaft span into said first shaftspan, said damped bearing means being adapted to damp whirling motion ofsaid first shaft span and of said second shaft span, said damped bearingbeing the only damped bearing means for damping whirling motion of saidspans.
 2. The bearing and support arrangement according to claim 1wherein said damped bearing means is an oil damped bearing meansincluding an annulus of oil, and said end of said first shaft span isfully supported by said annulus of oil.
 3. The bearing and supportarrangement according to claim 1 wherein said damped bearing means isaxially spaced from said nondamped bearing means.
 4. The bearing andsupport arrangement according to claim 1 wherein said first and secondshaft means are gas turbine engine shafts.
 5. The bearing and supportarrangement according to claim 1 wherein said second shaft span has afree end, and is hung at its free end from said first shaft span throughsaid nondamped bearing means.
 6. In an engine including a low rotorshaft and a high rotor shaft, the high rotor shaft being concentric withthe low rotor shaft and outwardly spaced therefrom forming an annularpassageway therebetween, the high rotor shaft having a span with a freeend and the low rotor shaft having at least one span, a bearing andsupport arrangement for the low and high rotor shafts comprising:nonrotating support means; damped bearing means disposed on said supportmeans, the end of the low rotor span being rotatably disposed on saiddamped bearing means; nondamped bearing means positioned within theannular passageway, said low rotor span being rotatablY disposed on saidnondamped bearing means, and the free end of the high rotor span beingrotatably disposed on said nondamped bearing means and hung from saidlow rotor span therethrough, said nondamped bearing means adapted totransmit whirling motion of said high rotor span into said low rotorspan, said damped bearing means adapted to damp whirling motion of saidhigh and low rotor spans, and said damped bearing means being the onlydamped bearing means for damping the whirling motion of both spans. 7.The bearing and support arrangement according to claim 6 wherein saiddamped bearing means is an oil damped bearing means including an annulusof oil, and said end of said low rotor span is fully supported by saidannulus of oil.
 8. The bearing and support arrangement according toclaim 6 wherein said low rotor shaft has a critical speed within itsoperating range and said high rotor shaft has a critical speed withinits operating range, and said damped bearing means is the only dampedbearing means for damping the whirling motion of said high and low rotorspans at said critical speeds.
 9. In an engine including a low rotorshaft and a high rotor shaft, the high rotor shaft being concentric withthe low rotor shaft and outwardly spaced therefrom forming an annularpassageway therebetween, the low rotor shaft having a span with a freeend and the high rotor shaft having at least one span, a bearing andsupport arrangement for the low and high rotor shafts comprising:nonrotating support means; damped bearing means disposed on said supportmeans, the end of the high rotor span being rotatably disposed on saiddamped bearing means; nondamped bearing means positioned within theannular passageway, said high rotor span being rotatably disposed onsaid nondamped bearing means, and the free end of the low rotor spanbeing rotatably disposed on said nondamped bearing means and hung fromsaid high rotor span therethrough, said nondamped bearing means adaptedto transmit whirling motion of said low rotor span into said high rotorspan, said damped bearing means adapted to damp whirling motion of saidhigh and low rotor spans, and said damped bearing means being the onlydamped bearing means for damping the whirling motion of both spans. 10.The bearing and support arrangement according to claim 9 wherein saiddamped bearing means is an oil damped bearing means including an annulusof oil, and said end of said high rotor span is fully supported by saidannulus of oil.
 11. The bearing and support arrangement according toclaim 9 wherein said low rotor shaft has a critical speed within itsoperating range and said high rotor shaft has a critical speed withinits operating range, and said damped bearing means is the only dampedbearing means for damping the whirling motion of said high and low rotorspans at said critical speeds.